Fly any plane:
Hold [Ctrl] + [Alt], then select the "Quick Mission" option. Keep
the keys pressed until the flight begins.
Atomic Moth superplane:
Hold [Right Alt] + [Right Ctrl] + [Right Shift] while selecting
any mission mode. The Atomic Moth will appear as an available
aircraft. Any mission in campaign mode may also be selected if
this is done when selecting "Play Single Mission" mode.